feat: make "only the app publishes a port" a binding rule

A published database port puts the data on the public internet, and Docker's
own iptables rules bypass ufw — the server looks firewalled while the port is
open. Previously this was only a comment in compose.yaml, which an agent adding
a db service later is free to ignore.

Anchored at every point where a service could be added or shipped:
- rules/security.md: the rule, the reasoning, and the audit commands
- rules/backend.md + architecture SKILL: constraint when picking the data layer
- agents/backend-dev.md: non-negotiable for any service it adds
- agents/qa-engineer.md: a published port is a Critical finding
- skills/deploy: blocking pre-flight audit
- compose.yaml: db example now uses expose:, with depends_on healthcheck
- docs/DEPLOYMENT.md + README: SSH tunnel instead of publishing for debugging

.claude/agents/backend-dev.md

@@ -36,6 +36,10 @@ The non-negotiables:
   as a fallback
 - Every new env var lands in `.env.example` in the same change
 - Whatever you add must run inside the Docker Compose stack
+- **Any service you add to `compose.yaml` gets no `ports:` entry.** Only `app`
+  publishes a port. A database, cache or queue is reached by service name over the
+  compose network (`db:5432`) and must stay unreachable from the host. Publishing it
+  exposes the data to the internet and bypasses the server's firewall.
 
 ## Tests
 For each route handler, write a Vitest test next to it